Solid state imagers, including charge coupled devices (CCD) and CMOS imagers, have been used in photo imaging applications. A solid state imager circuit includes a focal plane array of pixel cells, each one of the cells including a photosensor, which may be a photogate, photoconductor or a photodiode having a doped region for accumulating photo-generated charge.
During the manufacture of solid state imagers, the creation of defective pixels is unavoidable. These defective pixels, if not corrected, can cause severe degradation of image quality and, as a result, decrease the yield of parts during production. Thus, minimization of these defects during fabrication will yield a higher quality product. However, it is usually less expensive to make a device (e.g., semiconductor imager device) using less precise manufacturing tolerances. Devices that are produced using less precise manufacturing tolerances, on the other hand, have a higher probability of defects. Typical semiconductor fabrication rules define some tradeoff between the quality (lack of defects) and cost of manufacture. The manufactured semiconductor devices are tested for defects, and any semiconductor device having more than a certain percentage of defects is usually discarded.
Image acquisition semiconductor devices are especially sensitive to defects. A bad pixel in an imaging semiconductor will show up as a bad area on the acquired. The defective pixels may not work at all or, alternatively, may be significantly brighter or dimmer than expected for a given light intensity. Depending on the desired quality and the intended application, a single defective pixel may sometimes be sufficient to cause the device containing the pixel to be discarded.
In most instances, however, a small percentage of defective pixels can be tolerated and compensated for. Numerous techniques exist for locating and correcting defective pixel in a semiconductor imager device.
One simple technique for single defective pixel correction involves taking a signal from each pixel and storing the pixel values in memory. During image processing, the saved value for a defective pixel can be replaced by the average signal value of the neighboring pixels. These simple methods, however, are not viable for all pixel defects, for example, those suffering from excessive dark current. Other more complicated methods have been devised that can also correct defective pixels, including dark current pixels. For example, see the method discussed in the paper submitted by B. Dierickx and G. Meyanants “Missing Correction Method for Image Sensors,” submitted for Europto-SPIE/AFPAEC May 18-21, 1998.
Correction of multiple defects in a small area of an array, termed cluster defects, however, still remain a significant challenge. Accordingly, there is a need and desire for a method of correcting defective pixel clusters to improve the yield of imager manufacturing.